1. Field of the Invention
The present invention relates to a semiconductor device, a method and apparatus of manufacturing the semiconductor device, and particularly to a semiconductor device having an electrode structure connected by lead-free solder bumps, a method of manufacturing this semiconductor device, and a manufacturing apparatus used in this semiconductor manufacturing method.
2. Description of the Related Art
Accompanying the increased functionality and increased density of semiconductor devices, there has been an increasing number of semiconductor packages connecting a multi-pin semiconductor chip to a package substrate by solder bumps, and semiconductor packages having ball grid array (BGA) type external electrodes. In electrodes of this type of semiconductor chip, the surface boundaries of bonded sections vary in composition due to reactions between metals caused by heat history at the time of assembly, heat history at the time of mounting the semiconductor package and high temperature states and temperature changes in the usage environment. This may cause degradation in reliability, and one important element in addressing this problem is to select respective materials in order to give a metallic composition allowing reliability to be kept.
In order to do this, as shown in FIG. 5, it is common practice, when using solder composed of tin and lead as bumps, to use nickel or copper in a UBM (under bunks metal) layer 5, and to carry out bonding using this UBM layer 5 formed to a film thickness of at least 5 μm. In the case, of a nickel layer, tin and nickel in the solder react to form an intermetallic compound 11 by which the bump is bonded. In the event of a copper layer, an interface forms an intermetallic compound 11 of the tin and copper and bonds.
The reactivity of copper and tin is higher than that of nickel and tin, but in either case in the fused state at the time of bonding, and also under a temperature environment after bonding, a diffusion reaction is promoted and tin constituting the solder erodes the nickel or copper layer that is the UBM layer 5. As a result, there are problems such as tin being consumed at the bonding interface forming areas where lead density is high, and Kirkendall voids due to diffusion of tin, which are likely to reduce strength. In order to solve these problems, currently, a method of forming a thick layer of copper or nickel is used, or high melting point lead-rich solder with reducing tin content is used.
However, in recent years there have been moves towards lead free solder, mainly due to environmental problems, making it necessary to use solder having tin as the main component. With this type of solder with tin as a main component, in the case of using copper or nickel as the UBM layer 5 the above described problems are more evident, and seriously affect reliability.
Generally, in order to solve these problems, from the standpoint of solder wettability and mechanical characteristics, there is used solder having silver, bismuth, antimony or zinc added to the main component of tin, or solder that is a multiple component alloy having elements for preventing eating-away and diffusion of the UBM layer 5 formed from copper and nickel, added to silver, bismuth, antimony or zinc.
However, multiple component solder is generally supplied as solder pastes or balls to each electrode, which means that keeping microscopic additional elements uniform in the composition of each electrode is difficult. Keeping uniformity of microscopic additional elements would increase manufacturing costs.
Also, a result of having additional elements for preventing eating away and diffusion is that additional elements are previously dissolved in the tin that is the main component, and melting or solid-dissolution of the UBM layer 5 into the solder at the time of bonding stops at the lower limit, but the total amount of the UBM layer 5 dissolved varies depending on the bonding temperature. In order to bond stably it is necessary to increase the temperature, normally excessively dissolving the UBM layer 5. In order to prevent this phenomenon of the UBM layer being excessively dissolved it is necessary to include a lot of additional elements, but as a result the melting point becomes high and thermo stability at the time of manufacture must be taken into consideration, which poses serious questions for product design.
Also, as shown in FIG. 15, a reflow furnace for melting solder and making bonding sections is a system for passing through areas for preliminary heating, and actual heating and cooling that are temperature controlled by infra red rays or hot air, at a fixed speed using a conveyor, but with this system temperature control is difficult as the device is passed by the conveyor through each area, it is not possible to form an intermetallic compound of the bonding sections as was intended by the material design, and because of vibration of the conveyor while transporting within the furnace there is damage to a semiconductor wafer or chip, with solder bridges arising due to movement of the formed solder causing reduction in the product yield.
Japanese Patent Application Unexamined Publication No. 9-36120 discloses a solder bump electrode structure allowing enhanced bonding strength by preventing an intermetallic compound from being formed by diffusion of the solder and barrier metal. More specifically, a barrier metal layer, a first UBM layer, a second UBM layer, and a solder bump are laminated in this order on a bonding pad of the semiconductor chip. The first UBM layer is allowed to be alloyed with material of the solder bump. The second UBM layer includes metal that is also included in the solder bump and not allowed to be alloyed with the barrier metal layer, wherein the concentration of the metal in the second UBM layer is higher than that of the same metal in the solder bump.